1. Field of the Invention
This invention relates to a spread spectrum transmission system and more particularly, to a modulation and demodulation system of simple construction for spread spectrum transmission which is capable of suppressing considerably a variety of jamming, interference or noise unintendedly mixed in a spread spectrum signal received through transmitting means or recording and reproducing mediums and is applicable to preferably packaged media or to broadcasting networks and the like.
2. Description of the Related Art
In the field of information handling, it is important to increase the volume of information to be processed under a given condition. Recently, much effort has been paid to the technology of encoding, modulation and demodulation of information signals in this regard. For example, the multi-level QAM (quadrature amplitude modulation) system was applied to mobile communications for the purpose of increasing the transmission capacity per given frequency bandwidth. Although the number of levels of the QAM was increased from 16 to 256 (16-level QAM to 256-level QAM), in this trend, a further increase to 2.sup.16 (=65536) is conceivable but it seems infeasible because of its system complexity. As an example, if the information handling capacity of the 256-level QAM system was attempted to expand 1.5 times, a 4096-level QAM system would be needed. Therefore, in view of such situations, further innovative study and development will be required in the field of encoding and related modulation and demodulation technology in order to increase the volume of information which can practically be handled.
One known answer to this, is a spread spectrum (abbreviated to "SS") modulation and demodulation system. In this SS modulation and demodulation system, an information signal or the like is SS-modulated on a modulation (transmission) side by a broadband pseudo noise spread code (PN code) so that the signal is spread over a very wide frequency band, and on a demodulation (receiving) side, the received signal is despread by a PN code equivalent to that used on the modulation side. The advantage of the so-called SS communication system based on the foregoing modulation and demodulation technique includes security of communication, high resistance to external interference or noise and to intentional jamming, transmission spectrum compatibility with other communication systems, small transmission power, and high degree of multiplexing information to be transmitted (attained by changing the PN code). Therefore, the SS modulation and demodulation system is increasingly applied nowadays in various field of, for example communication equipment and consumer apparatuses.
The operation principle of the SS modulation and demodulation system or the SS communication system is that in order for suppressing interference (noise or jamming) which is unintendedly mixed in the path between a modulation section (on a transmission side) and a demodulation section (on a reception side), a local noise signal (for cancelling purposes) is produced from a demodulated output signal including the information and the interference components, and arithmetic processing is performed between the demodulated output signal and the local noise signal to restore only the information signal. SS modulation and SS demodulation processings are performed in the modulation section and the demodulation section, respectively.
FIG. 1 shows a fundamental configuration which is in the prior art. In this drawing, 2 and 3 are multipliers, 8 and 9 PN code generating circuits, 10 a modulation section, 11 a low-pass filter (LPF), 17 a substractor, 20' a demodulation section, and 21 a high-pass filter (HPF). Although not included in any practical system, an adder 15 is illustrated for the convenience of describing undesired interference mixed in the transmission path.
Let d(t) represent an information signal and P(t) a PN code. The PN code is usually a pseudo-noise signal. In the modulation section 10, spreading (modulation) is performed by the multiplier 2, resulting in a modulated signal D.sub.SS [=d(t).multidot.P(t)]. Letting I(t) represent undesired noise mixed in while the signal is passing through a transmitting path or recording mediums, the input signal to the demodulation section 20' is expressed by d(t).multidot.P(t)+I (t). In the demodulation section 20', despreading (demodulation) is performed by the multiplier 3 using a PN code equivalent to that used in the modulation section 10.
The resulting despread output signal is EQU [d(t).multidot.P(t)+I(t)].times.P(t)=d(t).multidot.P.sup.2 (t)+I(t).multidot.P(t) (1)
Since P(t) is the code taking only a value of either 1 or -1, p.sup.2 (t)=1. Accordingly, the despread output signal becomes d(t)+I(t).multidot.P(t), that is, there is obtained a mixture of the demodulated information d(t) and the spread noise I(t).multidot.P(t) (produced as noise being spread).
The output signal obtained is passed through the LPF 11 of optimum response, resulting in a demodulated output signal d(t)+n(t) (the foregoing is based on a typical conventional configuration).
On the other hand, if the foregoing signal is passed through the HPF 21 whose cutoff frequency is equal to that of the LPF 11, the output signal of the HPF 21 becomes I(t).multidot.P(t)-n(t). This HPF output signal is applied to a noise processor 25 of the next stage in which the -n(t) part is substantially suppressed, leaving virtually I(t).multidot.P(t) component. Accordingly obtained local spread noise (for cancelling purposes) I(t).multidot.P(t) is applied to the subtracter 17 in which it is subtracted from the despread output signal d(t)+I(t).multidot.P(t) so that the spread noise is cancelled out, resulting in only the information d(t) virtually (this is based on an improved conventional configuration). The noise suppressing characteristic of the demodulation section 20' of the conventional system is illustrated by the curve (a) in FIG. 10.
Since the foregoing operation principle is based on subtraction calculation, it is important to improve the accuracy of the local noise produced. That is, the jamming noise would be cancelled out completely if the waveform, phase and amplitude of the local spread noise were exactly identical with those of the jamming noise. However in reality, a suppression of more than 10 dB can hardly be attained as there is a certain difference between the spread jamming noise and the local spread noise which may be observed on an oscilloscope screen or the like.
Since the foregoing processing is performed over a very wide frequency band, a broadband circuit technology is also required. Since the same frequency range is used commonly by multiple stations because of its nature of the SS communication system, mutual interference is more or less inevitable, thus the performance of one SS communication is degraded if the power of interfering station is very high. In this regard, the reception signal quality can be improved if the power of the transmitting station is increased, but this will cause an increase of interference with other SS communications which can not be disregarded. The conventional system is effective only on SS interference of which the transmission form is known, but not on random noise nor the SS interference of unknown form. To deal with a plurality of SS interference of known forms, there is needed a combination of adders and loop circuits including a plurality of despreading demodulators, narrow-band filters of different passbands, and spreading modulators, this making the circuit configuration very complicated and increasing the system cost.